1. Field of the Invention
This invention relates to the techniques for the analysis of fluid-borne particles and particularly for looking at the asymmetry of such particles. For example, in the study of aerosols, aerosol dispersions and airborne particulate pollution control, there is a requirement for the rapid determination of particle size distribution especially in the diameter range 1 to 10 microns, together with some knowledge of the geometry and symmetry of individual particles. The latter information could, for example, enable particles with spherical symmetry to be identified and thus allow the counting/monitoring of liquid droplets in an environment including other solid, non-spherical particles. In the context of the present specification, the term particles is intended to apply both to solid bodies and to drops of liquid.
It is desirable for such techniques to be able to count individual particles in a sample at rates of, typically, 20,000 particles per second, to be able to distinguish between spherical and non-spherical particles in the sample and to count each type. Another desirable feature is to categorise spherical particles having diameters of 0.5-15 microns into a number of size bands and also in this connection to classify particle coincidences as `non-spherical` and hence to ignore them in the compilation of size spectra.
2. Discussion of Prior Art
The normal techniques for the examination of particles, as used in several instruments available commercially, employ the detection and analysis of electromagnetic radiation scattered by the particles. All such instruments use a mechanical mechanism to drive the sample air through a "sensing volume" where the carried particles are illuminated by the incident electromagnetic radiation. The radiation scattered by the particles is received by one or more detectors which convert the engergy to electrical signals from which information may be extracted by appropriate electrical circuits.
One class of instrument available commercially permits the collection of scattered radiation from large numbers of particles simultaneously, and uses this information to determine a mean figure for particulate mass per unit volume of gas or air, or the statistically averaged size distribution of particulate matter. These instruments are not capable of examining individual particles, and therefore cannot yield accurate particle counts or information relating to particle morphology.
A second class of instrument uses the properties of laminar flow in gases to restrict the particles to a smaller sensing volume and then, by focusing the incident electro-magnetic radiation in some way, is capable of the examination of individual particles, yielding a particle count and possibly approximate size distribution.
The prior art instruments, therefore, will give, to a certain extent, information on particle size and particle count. However, there is no instrument available that is capable of giving information on the asymmetry of individual fluid-borne particles.
There is therefore a need for a particle analyser which can analyse individual fluid-borne particles and give information as to the assymmetry of the particles by, for example, ascribing an asymmetry factor to the individual particles.